1. Field of the Invention
This invention relates to a subtraction processing method for radiation images, and more particularly to a method and apparatus for carrying out digital subtraction for radiation images by use of stimulable phosphor sheets.
2. Description of the Prior Art
Conventionally, a digital subtraction method is used for processing radiation images. In the method, two radiation images recorded under conditions different from each other are photoelectrically read out to obtain digital image signals, which are then subjected to a subtraction processing with respect to the corresponding picture elements of the images, thereby to obtain a signal for forming an image of a specific structure contained in the radiation images. The method makes it possible to reproduce a radiation image of only the specific structure by use of the thus obtained signal.
Basically, the subtraction processing is classified into the so-called temporal (time difference) subtraction processing method and the so-called energy subtraction processing method. In the former method, an image of a specific structure is extracted by subtracting the digital image signal of an X-ray image obtained without injection of a contrast media from the digital image signal of an X-ray image in which the image of the specific structure is enhanced by the injection of a contrast medium. In the latter method, an object is exposed to X-rays having energy distributions different from each other to obtain two X-ray images respectively containing the images of a specific structure recorded on the basis of the intrinsic X-ray energy absorption characteristics of the specific structure. Then, the two X-ray images are weighted appropriately, and subjected to subtraction to extract the image of the specific structure.
Since the subtraction processing is extremely effective for diagnostic purposes in image processings for medical X-ray photographs, it has recently attracted much attention, and research has continued to develop improved methods by use of electronic technology. The processing technique is particularly called digital subtraction processing method or, normally, digital radiography (abbreviated as "DR").
As the DR systems, there have heretofore been known digital fluorography using an X-ray fluoroscopic camera comprising a combination of an image intensifier tube (I.I. tube) and a television camera, which is categorized as the aforesaid time difference subtraction processing method, and scanned projection radiography using a line sensor, which is categorized as the aforesaid energy subtraction processing method.
In the digital fluorography, since an I.I. tube and a television camera are used, the image quality (particularly resolving power) in the scanning systems of these components is low. Further, since many signal conversion systems are used, the digital fluorography exhibits very low image quality and is practically unsatisfactory for obtaining detailed information of radiation images. The digital fluorography is also disadvantageous in that, since the recordable picture size is limited by the size of the light receiving face of the I.I. tube, the digital fluorography cannot be used for large objects.
In the scanned projection radiography, a line sensor is moved along an object and, at the same time, the energy level of the X-ray source is alternately changed for each scanning line so as to divide one image into two stripe-like components of high and low energy levels, respectively. The respective image signals thus obtained are then subjected to subtraction processing. Accordingly, the image resolving power decreases at least to half, and the scanned projection radiography exhibits very low image quality as in the case of the aforesaid digital fluorography.
Radiation images used for diagnostic purposes, such as those recorded on medical X-ray photographs, are required to exhibit particularly high resolving power and high image quality with respect to the image density and contrast. Although the aforesaid DR is a revolutionary technique from the viewpoint of the subtraction processing, it is practically unsatisfactory, and a need exists for drastic improvements thereof.